Use Keystone trusts to get fresh token

Instead of using user's auth token (which can expire) for interactions with
other services engine creates Keystone trust that impersonate user and
create new tokens on demand.

Heat stack is created on deployment start using token rather than trust so that
Heat could establish trust of its own (trusts cannot be chained).

New behavior is disabled by default and can be enabled using [engine]/use_trusts = True in murano.conf.
With trusts enabled engine will not work with Heat prior to Juno.
For Heat stacks with deferred actions or long deployment time to work it is also required to turn on trusts in Heat itself.
This can be done via [DEFAULT]/deferred_auth_method=trusts in heat.conf and ensuring that current user
has heat_stack_owner role (or any other that is in [DEFAULT]/trusts_delegated_roles=trusts in heat.conf)

Change-Id: Ic9f3f956ddb6ff2a300a08056ee841cf3c0db870
Implements: blueprint auth-for-long-running-requests
Merge "Use Keystone trusts to get fresh token"